Physitemp Instruments BAT-12 - Manual (Page 6)

6
6.1 The thermocouple is a simple and widely accepted device for measuring tem-
perature. It comprises two wires of dissimilar metals fused together to form a junc-
tion which produces an electrical output proportional to temperature. The National
Institute of Standards and Technology (NIST Monograph 125, 1974) has tabulated
the voltage/temperature relationships of many commonly used thermocouple pairs;
their tables on Copper-Constantan form the basis for calibration of Physitemp
thermometers.
6.2 At one time, accurate thermocouple temperature measurements needed elabo-
rate potentiometers and reference to a source of known temperature, such as an ice
bath. The advent of modern solid state devices has made possible the design of an
inexpensive thermocouple thermometer which is direct reading. The first of these
was Bailey thermometer BAT-4, which was designed in 1969 and is now in use
throughout the world. Your BAT-12 is an advanced version of the original equipment
using the latest low power digital technology and compensated electronic reference
circuitry.
6.3 As compared with thermistor sensors which were formerly used exclusively in
portable thermometers, thermocouples have these advantages:
(a) Wide temperature range, e.g. -200°C to over +1300°C.
(b) High stability of output.
(c) Interchangeability - no recalibration required.
(d) Accuracy traceable to NIST calibrations.
(e) Low cost; users can even make their own sensors.
(f) Microscopic size when needed, as in Physitemp microprobes.
(g) Nearly instant response.
(h) Better measurement accuracy due to low mass with smaller heat loss.
6.4 The main disadvantage of the thermocouple, low sensitivity, was overcome by
the development of auto zeroing amplifiers which are now used in all Physitemp
thermometers. This type of amplifier is essentially drift-free. It makes possible an
electronic thermometer which is permanently calibrated, just like a mercury ther-
mometer. The following notes may help the user to avoid some of the errors most
frequently made in temperature measurement.
6.5 Faulty measurement technique with any type of thermometer can produce
errors of several degrees. Errors attributed to "out of calibration" equipment can
often be corrected by a simple change of technique.
6.0 TEMPERATURE MEASUREMENT WITH THERMOCOUPLE SENSORS